Miniaturized high current-carrying capacity current-limiting fuses with built-in indicating plungers



Nov. 24, 1959 F. J. KOZACKA 2,914,636

MINIATU ED HIGH CURRENT-CARRYING CAPACITY CURR T-LIMITING FUSES WITHBUILT-IN INDICATING PLUNGERS Filed May 8, 1958 a- MINIATURIZED HIGHCURRENT-CARRYING CA- PACITY CURRENT-LIMITING FUSES BUILT-IN INDICATINGPLUNGERS WITH il 'federick J. Kozacka, South Hampton, N.H., assignor toThe 'Chase-Shawmut Company, Newburyport, Mass. Application May 8, 1958,Serial No. 733,945 4 Claims. (Cl. 200-121 This invention relates tominiaturized high currentcar'rying Capacity current-limiting fuses,particularly fuses adapted for the protection of individual cells ofger- 'manium rectifiers and of silicon rectifiers.

To achieve the degree of compactness required for such applications, iefor subminiature power fuses, the

ratio of the kva. rating of the fuse to the internal volume of the fusemust be very large. The required degree of compactness can only beachieved by providing such fuses with fusible elements in the form ofwide, extremely short ribbons. The are formed incident to blowing of thefuse can be quenched before the fusible element is entirely consumed ifthe latter is made of silver whose vapors are a bad conductor ofelectricity at the temperature range around the boiling point of silver,and if ip'lungers and it is, therefore, one object of this invention toprovide current-limiting fuses of the 'afore-mentioned character whichcomprise built-in indicating plungers.

Indicating plungers are normally held in position against the bias of aspring by a fusible element in wire form, generally made of steel, toobtain the highdegree of tensile strength required for the purpose. Thefusible element formed by the steel wire shunts the fusible silverribbon which normally carries substantially the entire current of thecircuit controlled by the fuse. The resistance of the steel wire is sohigh that it carries hardly any current as long as the fusible silverribbon is intact. After destruction by fusion and arcing of the currentpath formed by the silver ribbon, the voltage prevailing across thesteel Wire becomes sufficiently high to cause a current to flow capableof rapidly fusing the steel wire. The voltage gradient along the areformed by fusion and back-burning of the steel wire tends to berelatively small, the vapors of steel being a relatively good conductorof electricity at the temperature around the boiling point of steel.Because the voltage gradient along an arc resulting from vaporization ofsteel wire, even if relatively thin, tends to be relatively low, therequired minimum length for the fusible indicatingplunger restrainingwire of steel may exceed the maximum length permissible forthecurrent-carrying"ribbonfuse link of silver, i.e. the length not tobe'exceeded if the required current-carrying capacity is to bemaintained.

The prior art method to cope with substantially different requirementsin regard to the length of the current-carrying fusible element'and inregard to the length of the fusible restraining wire for a blown fuseindicating plunger consists in housing the former and the latter in twoseparate casings-of which one may be mounted on the other such as shown,for instance, in Figs. 1 and 2 United States Patent of United StatesPatent 2,794,095 to Frederick J. Kozacka, May 28, 1957, Striker PinStructures. This arrangement is, however, too bulky for subminiaturepower fuses such as required for the protection of individual germaniumand silicon rectifier cells.

It is, therefore, one object of this invention to provide subminiaturecurrent-limiting power fuses sufiiciently compact to be used for theprotection of individual germanium and silicon rectifier cells whichfuses have a built-in rather than an additional external indicatingplunger.

Another object of the invention is to provide a subminiature power fusehaving a built-in indicating plunger wherein the abutment meansprecluding the indicating plunger from coming off the fuse structureupon fusion of the restraining wire for the plunger are arranged insidethe extremely compact casing of the fuse in such a way as not to beimpaired by arcing.

Another object of the invention is to provide subminiaturecurrent-limiting power fuses having indicating plungers and restrainingwires therefore inserted into the circuit without resorting to solderjoints.

Other objects and advantages of the invention will, in part, be obviousand in part appear hereinafter.

For a more complete understanding of the invention reference may be hadto the following detailed description thereof taken in connection withthe accompanying drawings wherein I Fig. 1 is substantially alongitudinal section along a fuse embodying this invention, the sectionbeing taken along 1-1 of Fig.2;

Fig. 2 is a section along 22 of Fig. '1;

Fig. 3 is-a top plan view of the structure shown in Fig. 1 seen in thedirection of the arrow R;

Fig. 4 shows a portion of the structure of Fig. l on a larger scale; and

Fig. 5 is anisometric view of the structure of Figs.-1 to 3 upon blowingof the fuse showing the indicating parts in their indicating position.

Referring now to the drawing, numeral 1 has been applied to indicate atubular synthetic-resin-glass-fibercloth laminate casing. The binderresin for the laminate may either be a-melamin resin, a silicone resin,or some other synthetic resin suitable for the purpose in hand. A pairof metal plugs 2 is press-fitted into casing 1, closing both ends of thesame. Each plug 2 comprises a blade contact 3 forming an integral partthereof. Each blade contact 3 extendsaxially outwardly from the plug 2of which it forms an integral part and each blade contact 3 subdividesthe axially outer surface of each plug 2 into two contiguoussubstantially semi-circular areas 4. The axially inner surfaces of plugs2 are each provided with two grooves 5 extending across the axiallyinner surfaces of the plugs 2 immediately adjacent the center thereof.In other words, the length of grooves 5 is almost equal to the diameterof plugs 2 and the ribbontype fusible elements 6 inserted into grooves 5have 'virtually the largest possible width which can be imparted to afiat fusible element housed in casing 1. Fusible elements 6 are eachformed by a strip of sheet silver having 'V-shaped'lateral incisions '7defining a neck 8 of short length and width forming virtually a pointheat source when fusible elements 6 are carrying current. Fusibleelements Gare made of-silver because'silver has the most favorable ratioof conductivity and fusing energy for current limitation. Fusibleelements 6 are secured in grooves '5 by solder joints (not shown) tominimize the electric resistance between elements and plugs 2.

Each plug 2- defines one ofa pair of coaxial boresx9 extending in adirection longitudinally of casing 1. .The

distance between the axis of thecasing land thefusible elements 6 hasbeen marked a and the distance between the axis of casing 1 and bores 9has been marked b. It is apparent that the latter distance is muchlarger than the former distance. This combines the feature of greatestpossible link width, or fusible element width, with that of a desirablelocation-of the blown fuse indicator mechanism as will be more apparentfrom what follows below.

Each bore 9 extends from one of the center regions of one of thesemi-circular areas 4 into the inside of casing 1. Resilient metal caps19 and 11 are inserted into bores 9, and firmly held therein byfriction. Cap is recessed in one of the semicircular areas 4 in thecenter region thereof and houses a helical biasing spring 12, anindicating plunger 13 and an indicating cap of metal. Plunger 13 and cap15 are biased axially outwardly by spring 12 and arranged in coaxialrelation with respect to cap 10 and spring 12. The axially inner end ofplunger 13 projects through the axially inner end of cap 10 into one ofthe bores 9 in One of the plugs 2. The axially inner end of plunger 13is flattened and perforated, and a fusible plunger restraining wire 14of steel is threaded through the perforation in plunger 13. Wire 14extends through the bore 9 immediately adjacent cap 10 and throughcasing 1 and through the bore 9 immediately adjacent to cap 11.

Cap 11 clamps the two ends of wire 14 against the surface of bore 9 intowhich cap 11 is press-fitted, thus establishing a good electricalpressure contact between wire 14 and plug 2 remote from the indicatingplunger 13. It will be apparent that the circuit of the steel wire 14shunts the circuit of the fusible silver elements 6 whose conductivityis high, and that this shunt circuit is established by reliable pressurecontacts only, i.e. without resorting to soldered joints or connections.Provision of the additional indicating cap 15 forming an integral partof plunger 13 is optional. its presence is desirable because itincreases, when brightly colored, the ease of checking whether or not agiven fuse has blown. As a general rule blade contacts 3 will be silverplated and form a contrary background for colored indicating cap 15,from whatever angle cap 15 should be viewed.

A filler 16 of quartz sand fills the main space inside of easing 1 aswell as the spaces inside of bores of plugs 2.

Upon fusion of links or fusible elements 6 as a result of an excesscurrent, an arc is kindled which takes the place of fusible element 6.This are is rapidly quenched by the action of the pulverulentarc-quenching filler 16. The increasing voltage drop across the arccauses an increase of current flow through steel wire 14, as a result ofwhich the latter fuses almost instantaneously. The excess length of wire14 over links 6 is suflicient to generate the arc voltage required tosafely extinguish the arc resulting from fusion of wire 14. The task ofextinguishing this are is particularly onerous not only because this areis one resulting from the vaporization of steel, but also because thespace wherein this are is formed has been pro-heated by the previouspower are resulting from fusion of fusible elements 6. The body 16 ofquartz sand in which the axially inner end of plunger 13 is immersed isa suificiently reliable and effective are quencher to precludevaporization of this end of the plunger which would be dangerous sinceit would entail formation of a critical amount of steel vapor.

Having disclosed a preferred embodiment of my invention it is desiredthat the same not be limited to the particular structure disclosed. Itwill be obvious to any person skilled in the art that many modificationsand changes may be made without departing from the broad spirit andscope of my invention. Therefore it is desired that the invention beinterpreted as broadly as possible, and that it be limited only asrequired by the prior state of the art.

I claim as my invention.

1. In a subminiature current-limiting power fuse the combination of atubular casing of insulating material circular in cross-section, a pairof metal plugs closing both ends of said casing, groove means extendingacross the axially inner surfaces of said pair of plugs immediatelyadjacent to the center thereof, each of said pair of plugs defining oneof a pair of coaxial bores extending in a direction longitudinally ofsaid casing and being spaced a greater distance from the axis thereofthan said groove means, ribbon link means inserted into said groovemeans conductively interconnecting said pair of plugs, a first resilientmetal cap inserted into one of said pair of bores at the axially outerend thereof, an indicator plunger and an indicator plunger biasingspring housed in said first cap, a fusible restraining wire attached tosaid plunger extending through said one of said pair of bores andthrough said easing into the other of said pair of bores, a secondresilient metal cap inserted into said other of said pair of bores andclamping said fusible restraining wire against said other of said pairof bores, and a body of pulverulent filler inside said casing and insidesaid pair of bores.

2. In a subminiature current-limiting power fuse the combination of atubular casing of insulating material cir cular in cross-section, a pairof metal plugs closing both ends of said casing, a pair of bladecontacts each forming an integral part of one of said pair of plugs eachsubdividing the axially outer surface of one of said pair of plugs intotwo contiguous substantially semi-circular areas, each of said pair ofplugs defining one of a pair of coaxial bores each extending from one ofsaid semicircular areas into the inside of said casing, ribbon linkmeans arranged parallel to the plane defined by said pair of bladecontacts conductively interconnecting said pair of plugs, a resilientmetal cap inserted into one of said pair of bores at the axially outerend thereof, an indicator plunger, an indicator plunger biasing springand an indicator cap housed in said metal cap, a plunger abutmentforming an integral part of said plunger arranged outside of said metalcap inside said one of said pair of bores, a fusible wire attached tosaid plunger abutment extending through said one of said pair of boresand through said easing into the other of said pair of bores, meansinserted into said other of said pair of bores for clamping said fusiblewire in position, and a body of pulverulent filler inside said casingand inside said pair of bores.

3. In a subminiature current-limiting fuse the combinaof a tubularsynthetic-resin-fiber-glass-cloth laminate casing circular incross-section, a pair of metal plugs each press-fitted into one end ofsaid casing, a pair of blade contacts each forming an integral part ofone of said pair of plugs and each subdividing the axially outer surfaceof one of said pair of plugs into two contiguous substantiallysemi-circular areas, a first relatively short relatively Wide fusiblemeans formed by a strip of sheet silver arranged inside said casingparallel to the plane defined by said pair of blade contacts andconductively interconnecting the axially inner surfaces of said pair ofplugs, a second relatively long relatively thin fusible means formed bya wire of steel arranged inside said casing and conductively securedwith the ends thereof to points of said pairs of plugs immediatelyadjacent to said semi-circular areas, a first relatively short body ofquartz sand enveloping said first fusible means, a second relativelylong body of quartz sand enveloping said second fusible means, a caprecessed adjacent the center of one of said semi-circular areas, ahelical biasing spring arranged inside of said cap, and an indicatingplunger under the bias of said spring arranged in coaxial relationinside said spring and said cap and projecting through the bottom end ofsaid cap into said second body of quartz sand, and said plunger having aflattened perforated end through which said second fusible means isthreaded.

4. In a subminiature current-limiting fuse the combination of a tubularsynthetic-resin-fiber-glass-cloth laminate 5 casing circular incross-section, a pair of metal plugs each press-fitted into one end ofsaid casing, a pair of blade contacts each forming an integral part ofone of said pair of plugs and each subdividing the axially outer surfaceof one of said pair of plugs into two contiguous substantiallysemi-circular areas, each of said pair of plugs defining one of a pairof coaxial bores each extending from one of said semi-circular areasinto said casing, a pair of relatively short ribbon fusible elements ofsilver arranged in planes parallel to the plane defined by said pair ofblade contacts each having one single point of reduced cross-sectionalarea between the ends thereof, a relatively long wire fusible element ofsteel extending with the ends thereof into said pair of coaxial bores, afirst metal cap inserted into one of said pair of bores at the axiallyouter end thereof, an indicator plunger, and indicator plunger'biasingspring and a second metal cap housed in said first metal cap, saidplunger projecting through said first metal cap into one of said pair ofbores and having a flattened perforated end situated in said one of saidpair of bores, said wire fusible element being threaded through saidperforated end of said plunger, and a body of quartz sand inside of saidcasing and inside of said pair of bores.

References Cited in the file of this patent UNITED STATES PATENTS2,648,739 Lebens Aug. 11, 1953 2,800,554 Dannenberg et al. July 23, 19572,834,852 Swain et al. May 13, 1958 FOREIGN PATENTS 580,462 GreatBritain Sept. 9, 1946

